A wide variety of parking and service brakes are known having separate braking surfaces to effect a brake application while others employ alternate techniques for independent actuation of the service brakes and parking brake on a same brake surface of a vehicle. One of the independent braking surfaces is typically prevented from rotating by being fixed to the vehicle and another braking surface is fixed to and rotates with a vehicle wheel, for example, as in the well-known drum-in hat braking mechanisms. Also, one of the braking surfaces may be fixed to the vehicle while another is fixed to a rotatable portion which is indirectly coupled to and rotates with a vehicle wheel, for example, as in brakes employing a flexible friction band which surrounds a drum fixed to a vehicle drive shaft. In all cases, however, wheel rotation is retarded when the auxiliary brake is applied. The auxiliary brake may be hydraulically actuated, spring actuated, actuated by a mechanical coupling such as a cable, or a combination of these techniques may be employed. Mechanical actuation of the auxiliary brake may be by way of a cable, and a foot depressible pedal or hand actuated lever, or the brake may be applied automatically when a shift lever is moved to the “park” position and released when that lever is moved away from the “park” position. Other brake applying and releasing controls may be employed. Auxiliary brakes may be embodied in a completely independent form, or may share components with the normal service brakes. Component sharing typically reduces overall cost. One common technique employs the use of an operator actuable cable and linkage for independently moving the service braking surfaces into and from engagement. In this common system, an automatic service brake adjuster may be operable when braking during backing to maintain a desired brake surface running clearance. In this case, periodic adjustment of cable length is the only other adjustment required. A preferred embodiment of the present invention is used in a mechanically operable drum-in-hat auxiliary brake having an independent actuator, adjuster, and brake shoes with the only shared component being a rotor-drum assembly. The techniques of the present invention may be applied to other types of braking systems.
The desirability of limiting braking forces has been addressed in the prior art as in U.S. Pat. No. 6,619,443 that discloses an overload protection device for a truck-mounted brake assembly on a railcar. Railroad car brakes employ pneumatic as well as manual devices for applying the same brake. If the pneumatic brake system is activated and then the manual device is also actuated, the forces on the brake assembly are multiplied and brake beam deflection or failure may result. The patented arrangement provides a preloaded compression member in a chamber to absorb excess force before deflection and possible failure of the brake beam occurs.
In the highway vehicular area, force limiting has also been addressed, but in complex systems dealing with the vehicle service brakes. Antilock or anti-skid braking systems are, of course, broadly brake force limiting systems as disclosed in U.S. Pat. No. 3,384,205 wherein a drum style service brake includes a yieldable abutment that permits circumferential displacement of both the primary and secondary brake shoes. Displacement is limited by a stop whereupon further increase in the brake actuating force results in only movement of the actuating piston associated with the secondary shoe so that braking increases with a trailing shoe action.
Despite the presence of warning lights and buzzers, drivers may still occasionally forget to release the parking brake and drive away with the brake still applied. This may result in excessive wear, overheating and to permanent damage to the parking brake mechanism. In particular, an auxiliary brake such as a high gain drum-in-hat parking brake may be subject to failure when an operator drives off without releasing the park brake. Brake shoes may buckle, anchoring structures may fracture, or other damage may result.
Current safety requirements require a park brake to hold a vehicle on a 30% grade and should an operator move a vehicle without releasing the park brake damage could occur by the introduction and development of excessive braking forces into the auxiliary braking system. U.S. patent application Ser. No. 10/711,049 provides structure that limits the developed of excessive braking force should a parking brake not be released prior to driving away. This structure normally functions in an adequate manner but the components are exposed to brake dust and the environment and may be effected thereby.